1. Field of the Invention
The invention relates to a steering system.
2. Description of Related Art
A rack that meshes with a pinion of a pinion shaft is formed on a rack shaft of a rack and pinion steering system described in Japanese Patent Application Publication No. 2012-188005 (JP 2012-188005 A). The pinion shaft is supported by a gear housing. The rack shaft is inserted into a through hole that runs through the gear housing. Both ends of the rack shaft are coupled to wheels through a ball joint, etc. The pinion and the rack convert the rotational motion of a steering wheel into an axial motion of the rack shaft. The rack shaft is pressed against the pinion shaft by a rack guide from the back of the rack shaft, or the opposite side of the rack shaft from the pinion shaft. A first fitting portion and a second fitting portion are provided in the through hole in the gear housing. A rack shaft bush is fitted into the first fitting portion, and a rack bush is fitted into the second fitting portion. The rack bush is disposed closer to the rack guide than the rack shaft bush is.
In the rack and pinion steering system described in JP 2012-188005 A, the rack bush fitted into the second fitting portion of the gear housing slides with the rack shaft when the rack shaft moves in an axial direction. Accordingly, the rack bush, for a long period of time, bears a pressing force against the gear housing, which is generated because the rack bush is sandwiched between the gear housing and the rack shaft, and a load such as a sliding force generated by the sliding friction between the rack bush and the rack shaft.
In the rack shaft, particularly, backlash is likely to occur at a portion pressed by the guide member such as the rack guide. Accordingly, the rack bush disposed closer to the guide member than the rack shaft bush is, has a function to absorb the backlash. When a vehicle travels on a rough road or a wheel runs onto a curb, and a large load such as reverse input is momentarily applied to the rack shaft through wheels, the large load is likely to be applied to the rack bush disposed close to the guide member.
If a long term load or a sudden large load is applied to the rack bush disposed close to the guide member, distortion or abrasion may occur, which causes deterioration of the rack bush.